1. Field of the Invention
The present invention relates to phosphors, more particularly a phosphor including semiconductor nanoparticles and conductive transparent compounds.
2. Description of the Background Art
In recent years, illumination devices including light emitting diode elements as the constituent component are attracting attention. A light emitting diode element has superior properties such as saving power, a long product lifetime, and low environmental load. Particularly, there is developed a light emitting device for various illuminations that can provide white light by combining a light emitting diode element emitting blue light or ultraviolet light with various phosphors excited by light emitted from the light emitting diode element and capable of luminescence at a desired wavelength. Such light emitting devices raise the expectation for illumination devices replacing incandescent lamps and fluorescent lamps.
Conventionally, rare earth activation phosphors have been used for the fluorescent material in light emitting devices. In order to enable the production of a light emitting device having high color rendering property and high luminance efficiency, the approach of employing semiconductor nanoparticle phosphors is now attracting attention. A semiconductor having an energy gap of the direct type exhibits phosphorescence based on a wavelength inherent to that substance. However, by restricting the particle size to approximately the level of the Bohr radius, the kinetic energy of the particles becomes discontinuous in both the balance band and conduction band, leading to a shorter emission wavelength in proportion to the particle size. Therefore, the emission wavelength can be controlled arbitrarily for a phosphor employing semiconductor nanoparticles (hereinafter, also referred to as semiconductor nanoparticle phosphor), differing from a conventional phosphor. Further, by combining a plurality of types of semiconductor nanoparticle phosphors, a light emitting device that produces light at various desired spectra can be obtained.
As conventional semiconductors serving as the material of semiconductor nanoparticles, research and reports have been provided mainly on CdSe (refer to C. B. Murray, D. J. Norris, and M. G. Bawendi, Synthesis and Characterization of Nearly Monodisperse CdE (E=S, Se, Te) Semiconductor Nanocrystallites, “Journal of the American Chemical Society”, 115, 1993, pp. 8706-8715) for semiconductors of the II-VI group compound, and on InP for semiconductors of the III-V group compound. Since a semiconductor nanoparticle is small in size, the ratio of surface constituting the particle is high. It is considered that defects at the surface induce non-radiative transition, leading to lower luminance efficiency. In view of such defects, each of the semiconductor nanoparticles is covered with a substance having a band gap greater than that of the semiconductor material such as zinc sulfide, achieving a core/shell structure for the semiconductor nanoparticle. Accordingly, the luminance efficiency is improved significantly. Furthermore, by protecting the outer side of the shell with a modified organic compound, sufficient luminance efficiency can be obtained.
A semiconductor nanoparticle protected with a modified organic compound is stable in liquid phase. However, semiconductor nanoparticles must be dispersed in a solid since it is not suitable for industrial application in the liquid phase.
For example, Japanese Patent Laying-Open No. 2006-282977 discloses the technique of dispersing semiconductor nanoparticles in a glass matrix to prevent reaction caused by oxygen and/or water to avoid degradation. Glass is suitable as the matrix material for phosphors since it is transparent. However, the luminance efficiency of semiconductor nanoparticles that is 70% in liquid will become as low as 30% after dispersion in glass. This luminance efficiency is a value defined as the ratio of the number of photons radiated as photoluminescence to the number of absorbed photons.
It is known that semiconductor nanoparticles dispersed on a metal substrate typically of gold has the luminescence intensity increased, as compared to the case where semiconductor nanoparticles are dispersed on a glass substrate (refer to Yuichi Ito, Kazunari Matsuda, and Yoshihiko Kanemitsu, Mechanism of photoluminescence enhancement in single semiconductor nanocrystals on metal surfaces, “Physical Review B”, 75, 2007, p. 033309 1-4). The luminescence intensity refers to the energy intensity of photoluminescence generated when exciting light of a predetermined energy is radiated. The relationship is represented by the following Equation (1).Luminescence intensity=intensity of exciting light×optical absorptance by phosphor×luminance efficiency xphosphor energy per photon/energy of exciting light per photon  Equation (1).
Increase in the luminescence intensity is an effect caused by the improvement in luminance efficiency in view of all the particles. Single semiconductor nanoparticles on a glass substrate have a luminescence intensity corresponding to a flickering phenomenon repeating ON and OFF over time, whereas semiconductor nanoparticles on a metal substrate do not exhibit a flickering phenomenon. Therefore, semiconductor nanoparticles on a metal substrate can exhibit stable and strong light emission, as compared to semiconductor nanoparticles on a glass substrate, when exciting light of the same luminescence intensity is employed.
The flickering phenomenon is considered to be caused mainly by the charge up of semiconductor nanoparticles due to electrons that are discharged outside, among the excitons generated in the semiconductor nanoparticles by photoexcitation (refer to Xiaoyong Wang, Xiaofan Ren, Keith Kahen, Megan A. Hahn, Manju Rajeswaran, Sara Maccagnano-Zacher, John Silcox, George E. Cragg, Alexander L. Efros and Todd D. Krauss, Non-blinking semiconductor nanocrystals, “NATURE”, 459, 2009, pp. 686-689). One difference between a glass substrate and a metal substrate is that the former is an insulator whereas the latter is conductive. Since the glass substrate is an insulator, the electrons discharged from the semiconductor nanoparticles dispersed on the glass substrate must jump over a high energy barrier in order to return back to the semiconductor nanoparticles. In contrast, the energy barrier is low on a metal substrate. The electrons discharged from semiconductor nanoparticles can readily return to the semiconductor nanoparticles. Since photoluminescence cannot be exhibited under the state where semiconductor nanoparticles are charged up, the luminance efficiency of the semiconductor nanoparticles dispersed on the glass substrate will be lowered.
It has been confirmed that luminescence intensity is increased by dispersing semiconductor nanoparticles on a metal substrate, as compared to the case where a glass substrate is used, as set forth above. However, gold possesses the property of absorbing and reflecting visible light, as well as being colored, causing the exciting light and/or fluorescence to be absorbed and scattered. Therefore, it is not preferable to use gold as the base substance directed to dispersing a phosphor in the production of illumination devices.